HOMOGENEOUS CATALYTIC-HYDROGENATION .6. SYNTHETIC AND MECHANISTIC ASPECTS OF THE REGIOSELECTIVE REDUCTIONS OF MODEL COAL NITROGEN, SULFUR, AND OXYGEN HETEROAROMATIC-COMPOUNDS USING THE (ETA-5-PENTAMETHYLCYCLOPENTADIENYL)RHODIUM TRIS(ACETONITRILE) DICATION COMPLEX AS THE CATALYST PRECURSOR

The synthetic and mechanistic aspects of the regioselective hydrogenation of representative mono and polynuclear heteroaromatic nitrogen, sulfur, and oxygen model coal compounds such as 2-methylpyridine (1), N-methylindole (2), benzofuran (3), benzothiophene (4), quinoline (5), 2-methylquinoline (6), 5,6- and 7,8-benzoquinolines (7 and 8), and acridine (9) were studied with a (eta-5-pentamethylcyclopentadienyl)rhodium tris(acetonitrile) dicationic complex, [Cp*Rh(CH3CN)3]2+, as the catalyst precursor. The order of relative rates as a function of structure was found to be 8 >>> 9 > 5 > 7 > 6 > 4 >> 1-3. Competitive hydrogenation experiments of 5 with other model coal compounds, 1-4, 6-9, pyridine (10), and isoquinoline (11) and its regioselective reduction product, 1,2,3,4-tetrahydroquinoline (12), showed the following effects on the initial hydrogenation rate of 5: no effect (2-4), enhancement (8), and inhibition (9-12). In addition, 7,8-benzoquinoline (8) and its reduced product, 1,2,3,4-tetrahydro-7,8-benzoquinoline (13), were found to also enhance the initial hydrogenation rates of 2, 6, and 7; a [Cp*Rh]2+-catalyzed transfer hydrogenation mechanism was invoked as one explanation for this rate enhancement by using 13-d5 in the presence of 5, which showed deuterium atom transfer to provide 12-d3. Replacement of H-2 with D2 provided information on several of the mechanistic aspects of these selective hydrogenation reactions and included reversibility in the N=C and C=C bond reductions of six-membered N heteroaromatic compounds (1, 5-9), stereoselective reduction of the C=C bond in the five-membered N, S, and O heteroaromatic ring compounds (2-4), and exchange of aromatic ring and 2-methyl group hydrogens (2-9). The catalytic hydrogenation precursor Cp*Rh complexes for ligands 1 and 5-9 had the known structural formulas [Cp*Rh(eta-1(N)-ligand)(CH3CN)2]2+, While those for 2-4 are speculated to have [Cp*Rh(eta-2-ligand)(CH3CN)2]2+ structures, eta-2 bonding of the C=C bond in the five-membered heteroaromatic ring to Cp*Rh. A partially hydrogenated pyridine intermediate, 1,2,5,6-tetrahydropyridine (14), bonded to [Cp*Rh]2+, [Cp*Rh(eta-1(N)-1,2,5,6-tetrahydropyridine)(CH3CN)2]2+ (15) was synthesized and reacted with D2 gas to provide further evidence for a possible intermediate in the selective hydrogenation process. Moreover, thermal dehydrogenation of 15 was observed with formation of a [Cp*Rh(eta-1(N)-pyridine)(CH3CN)2]2+ complex. High-pressure NMR experiments were able to further verify the above-mentioned mechanistic pathways with quinoline (5) as an example, but the identity of intermediate [Cp*RhH-quinoline]2+ complexes was not successful. An overall mechanism for selective N, S, and O heteroaromatic ring hydrogenation will be presented.